bcache_register(): don't bother with set_blocksize()

We are not using __bread() anymore and read_cache_page_gfp() doesn't
care about block size.  Moreover, we should *not* change block
size on a device that is currently held exclusive - filesystems
that use buffer cache expect the block numbers to be interpreted
in units set by filesystem.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christian Brauner <brauner@kernel.org>
ACKed-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>